Process for controlling the carbon content of a molten metal bath

ABSTRACT

In a top blown oxygen converter the carbon content of a molten metal bath is determined by measuring the amount of both carbon dioxide and unreacted oxygen in the waste gas. A phase change in the carbon-oxygen reaction process, believed to be an equilibrium phase change, occurs during the latter stages of the oxygenblowing process and is indicated by a simultaneous rapid decrease in the amount of carbon dioxide and a rapid and continuous increase in the amount of unreacted oxygen in the waste gas. The carbon content of the molten metal bath when this phase change occurs is substantially the same for all of the heats in a given converter or furnace that is supplied with oxygen at substantially the same rate. The carbon content of the moltenmetal bath when the phase change occurs is determined for several different heats and a constant is established for a particular furnace. This constant is the carbon content of a molten-metal bath at the time the carbon reaction process phase change occurs. In subsequent heats the amount of carbon dioxide and unreacted oxygen in the waste gas is continually measured, preferably during the later part of the conversion process, and the time at which the phase change occurs is noted. Thereafter, the carbon evolved from the converter as a gas is continuously measured, summated and subtracted from the carbon known to be present in the molten metal bath when the phase change occurred. When the carbon remaining in the molten metal bath reaches the desired carbon level for a particular grade of steel, the furnace is turned down and the conversion process terminated.

United States Patent [72] Inventor Reginald Wintrell Primary Examiner-L.Dewayne Rutledge Gibsonia, Pa. Assistant Examiner-G. K. White [2]] Appl.No. 792,588 Attorneys-Oscar B. Brumback, Stanley J. Price, Jr. and Olin[22] Filed Jan. 21, 1969 E. Williams [45] Patented Sept. 21, 1971 [73]Assignee Koppers Company, Inc.

--" ABSTRACT: In a top blown oxygen converter the carbon Content of amolten metal bath is determined by measuring the amount of both carbondioxide and unreacted oxygen in the i waste gas. A phase change in thecarbon-oxygen reaction process, believed to be an equilibrium phasechange, occurs [54] PROCESS FOR CONTROLLING THE CARBON during the latterstages of the oxygen-blowing process and is CONTENT OF A MOLTEN METALBATH indicated by a simultaneous rapid decrease in the amount ofschimsJnrawhlg Figs carbon dioxide and a rapid and continuous increasein the amount of unreacted oxygen in the waste gas. The carbon con- [52]US. Cl 75/60, tent of the molten metal bath when this phase changeoccurs i 23/230 substantially the same for all of the heats in a givenconverter [51] Int. Cl C2lc 5/32, or furnace that is supplied withoxygen at Substantially the 33/20 same rate. The carbon content of themolten-metal bath when [50] Fleldof Search 75/60; the phase changeoccurs is determined f several diff t 23/230 heats and a constant isestablished for a particular furnace. This constant is the carboncontent of a molten-metal bath at [56] References Cited t the time thecarbon reaction process phase change occurs. ln UNITED STATES PATENTSsubsequent heats the amount of carbon dioxide and unreacted 3,181,3435/1965 Fillon 73/23 oxygen in the waste gas is continually measured,preferably 3,236,630 2/1966 Stephan 75/60 during the later part of theconversion process, and the time at 3,329,495 7/1967 Ohta et al. 75/60which the phase change occurs is noted. Thereafter, the car- 3,377,l584/1968 Meyer et al..... 75/60 bon evolved from the converter as a gas iscontinuously mea- 3,432,288 3/1969 Ardito et a1 75/60 sured, summatedand subtracted from the carbon known to be 3,463,631 8/1969 Vayssiere etal.... 75/60 present in the molten metal bath when the phase change 0c-3,485,6l9 12/1969 Maatsch et al 75/60 curred. When the carbon remainingin the molten metal bath 3,489,518 l/197O Revell et al. 75/60 X reachesthe desired carbon level for a particular grade of steel. 3,520,6577/1970 Frumerman..... 23/230 the furnace is turned down and theconversion process ter- 3,528,800 9/1970 Blum et a1 75/60 minated.

0 \M V AIM/NV" I 30 70 IV Carbon Honax/dl porn/Han of carbon gun leaving"an! sa-- w 2J- r/ Larbon lllll/ in Mm k g 2 0 E i a s 8 5., I a 1.5-3.2, A30 i E 3 Oxygen percent in was sample gas I /.0 2a-, L l

' l l 1 l l a 2 4 s a l0 l2 l4 [6 w 20 BLDW TIME, minulas PATENTEDSEPZII971 /V\, \I II r W W \N\I\ 1 1B, 1 I 7 0 IV Carbon Monoxide percentageat aarbon gases leaving vessel I I I N 60- I Q 2.5- I/ Carbon level inbat/I I E l a E 2.0 -E I L E s E /.5 0. 3 n 30- E Oxygen percent inwaste sample gas [.0 I g 20- l V I -I 0.5 F Carbon Dioxide percent l inwaste sample I I #08 e" 0 I e-...-..-.-n" L "1' I I I I I I I 0 2 4 6 8l0 l2 l4 l6 I8 BLOW TIME, minutes 151 n 2/- 70 F1 5-. B 1' l8- J I I 2.5Q Carbon level in bat/I III I I5'- 508 1g m n I I l E g e a k I I t g/2--4o I Q El 5 n v/ (a: I a Q I percent In waste gas I Q l.5- R 9 n(Enlarged Scale) I E Q q I q Q Q 0 I U l I e 8 Oxygen percent I g A inIraete sample gae i s I a \a I l I Carbon Dioxide percent 7:171: I 3--10 in Ivaete sample gae I :2:

e' p fl ,1 a .I. on I 0 "--"I I l I I I I I 0 2 4 8 8 l0 l2 l4 l6 I8 20BLOW TIME, minute:

Ml VE/V TOR Attorney PROCESS FOR CONTROLLING THE CARBON CONTENT OF AMOLTEN METAL BATH BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention relates to a process for controlling the carbon contentof a molten-metal bath and more particularly to a process forcontrolling the carbon content of a molten-metal bath in a top blownoxygen converter by determining the carbon content of the molten-metalbath when a phase change occurs in the carbon reaction process andthereafter determining the carbon evolved as a gas from the converter.

2. Description of the Prior Art In a top blown oxygen converter oxygenis introduced into a molten-metal bath to oxidize the impurities andcarbon. A controlled amount of carbon is removed from the moltenmetalbath in gaseous form to produce steel containing a predeterminedpercentage of carbon. The control and measurement of the carbon contentof the molten-metal bath by the prior techniques used in open hearthtype furnaces is not possible with the top blown oxygen converterbecause of the rapidity at which the conversion takes place. Theextraction of samples of the molten metal during the conversion processfor carbon determination by chemical analysis has also provedimpractical.

Several proposals have been made in the past to continuously determinethe carbon content of a molten-metal bath in a top blown oxygenconverter. U.S. Pat. No. 3,181,343 determines the carbon content of themolten-metal bath at turndown by a continuous subtracting of thesummated carbon present in the waste gas from the total carbon in thematerial initially fed to the converter. This process has one majorinherent source of error. The total initial carbon content of thematerials is determined by calculating the saturation carbon level inthe hot metal and the carbon in the other constituents of the charge.Errors occur in sample analysis and the temperature of the hot metalwhich result in errors in the carbon content of the hot metal. It isalso difficult to assess the carbon content of varying grades of scrap,pig iron and the like. The magnitude and deviation of the summatederrors, although possibly small in comparison with the initial carbonlevel, are of the same magnitude and deviation to the final carbonlevel, thus making this process impractical for reliable low carbonsteel conversion processes.

Others have attempted to avoid the induced errors of the above processby eliminating the determination of the total carbon content of thematerial supplied to the converter. In U.S. Pat. No. 3,329,495 theamount of waste gas given off during the conversion process iscontinuously measured and the amount of carbon dioxide and carbonmonoxide present in the waste gas is also continuously measured. Thesemeasurements are correlated to determine the decarburizing velocityduring the conversion process and the decarburization velocity isdifferentiated to obtain the decarburizing acceleration. The times atwhich changes occur in the decarburizing acceleration are noted and thecarbon content of the molten metal bath is determined. The above processis dependent on the rate of carbon decarburization and the relativelysmall changes in both the velocity and acceleration must be carefullyand accurately measured, which is difficult, especially during theinitial stages of the conversion process.

In U.S. Pat. No. 3,377,158 the carbon oxidation rate is determined bymeasuring the carbon dioxide and oxygen in the waste gas and the amountof oxygen blown during the same period of time. From these measurementsthe carbon oxidation rate is calculated and a curve is establishedcorrelating the carbon oxidation rate with the carbon content of themolten-metal bath. The curve is extrapolated for the carbon oxidationrate to zero carbon and curves from previous blows are used to determinethe approximate quantity of carbon remaining in the molten-metal bathand thereafter a fixed quantity of oxygen is supplied to the bath toobtain molten metal with a desired carbon content.

In U.S. Pat. No. 3,326,630 the oxygen content of the waste gases ismeasured and the conversion process is terminated when the oxygencontent reaches a given value. It is stated the carbon content of themolten-metal bath is directly related to the oxygen content of the wastegas.

It will be apparent that the above processes require accurate andcontinuous measurement of both the volume and the composition of thewaste gases to determine the carbon content at any given period duringthe conversion process. It is difficult to obtain the above measurementswith the degree of accuracy required for low carbon steels.

SUMMARY OF THE INVENTION According to the present invention continuous,accurate volumetric measurement and accurate composition of the wastegas is not required to determine the carbon content of the molten-metalbath when a phase change occurs in the carbon reaction process. Thisphase change is determined by measuring both the amount of carbondioxide in the waste gas and the amount of unreacted oxygen present inthe waste gas. The phase change in the carbon reaction process takesplace. When there is a simultaneous rapid decrease in the amount ofcarbon dioxide and a rapid increase in the amount of unreacted oxygenpresent in the waste gas, the carbon content of the molten-metal bath atthe time of this phase change is substantially the same for a majorityof the beats and is not dependent on the total carbon in the initialcharge nor dependent on the rate of decarburization. With the knowncarbon content of the molten-metal bath determined at the time of thephase change, the amount of carbon removed thereafter is determined bysummating the carbon in the gaseous carbon compounds evolved as a gasand subtracting the summated value from the known amount of carbon inthe bath.

Accordingly, the primary object of this invention is to determine thecarbon content of a molten-metal bath without first determining thetotal carbon in the charge fed to the converter.

Another object of this invention is to control the carbon content of amolten-metal bath without continually and accurately measuring thevolume and composition of the waste gas.

A still further object of this invention is to provide a method ofaccurately determining the carbon content of a moltenmetal bath at apreselected time during the reduction process.

These and other objects and advantages of this invention will be morecompletely disclosed and described in the following specification, theaccompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphical representationof the change in the oxygen and carbon dioxide composition of the wastegas during the reduction process and illustrating the rapid change inthe amounts of unreacted oxygen and carbon dioxide at the time when thephase change occurs in the conversion process.

FIG. 2 is a graphical representation similar to FIG. 1 for a differentheat illustrating the change in the composition of the waste gas at thetime of the phase change and includes a graphical representation ofdifference between the amount of unreacted oxygen and the amount ofcarbon dioxide in the waste gas and the rapid increase in this value atthe time of the phase change.

DESCRIPTION OF THE PREFERRED'EMBODIMENT In accordance with the presentinvention a gas analyzer is adapted to receive and analyze the wastegases given off by the converter during the conversion process. Suitableapparatus for analyzing the waste gases is disclosed and described inU.S. Pat. No. 3,489,5 l8, entitled "Carbon Determination and Apparatus."It should be understood, however, other apparatus may be used that issuitable to determine the volume and composition of the waste gas. Thepercent carbon dioxide present in the, waste gas is periodicallymeasured and plotted.

Similarly, the percent unreacted oxygen in the waste gas is periodicallydetermined and plotted on the same graph as the percent carbon dioxidein the waste gas.

In order to determine the constant for the particular converter, thetime at which the percent carbon dioxide in the waste gas rapidlydecreases and the percent oxygen in the waste gas rapidly increases isnoted. Subsequent to the time when the phase change occurs, the carboncontent of the waste gas is continually measured and converted to a bathlevel until the termination of the conversion process. The steel iscarefully analyzed for carbon and the total carbon remaining in thesteel is determined. By back calculation according to the followingequation, the carbon content at the time of the phase change isdetermined:

(1) CF+CWG=CPC Where CF is the carbon content of the end product and CWGis a summation of the carbon evolved after the phase change was observedconverted to the equivalent bath carbon loss, CPC is a constantindicating the carbon content of substantially all molten-metal baths ina particular converter at the time the phase change occurs. Values ofCPC for several heats are measured to'obtain a statistically accurateaverage constant for subsequent heats.

Thereafter the CPC for the particular converter is employed as thestarting point for controlling the carbon content of the particularmolten-metal bath. Since the phase change in the carbon reaction processdoes not occur until the latter part of the conversion process, themeasurement of the waste gases for the early part of the blow is notnecessary. For example, the gas analyzer can be activated in aconventional B.O.F. process after the conversion process determined fromthe start of the oxygen blow has continued for about 12 to 14 minutes.Thereafter the gas analyzer periodically measures both the percent ofoxygen in the waste gas and the percent carbon dioxide in the waste gas.When the curves for oxygen and carbon dioxide simultaneously and rapidlycontinue to diverge, as illustrated in FIG. I, the carbon content of themolten metal bath is known. Thereafter the carbon content of the wastegases is carefully measured, summated and subtracted from the knowncarbon content and the conversion process is terminated when the desiredlevel of carbon is present in the molten-metal bath. The carbon contentat the termination of the conversion process CF is determined accordingto the following equation:

where ECWG is the summation of the carbon evolved in the waste gasescalculated for the actual bath carbon reduction between the time thephase change occurs TPC and the time the conversion process isterminated TF.

As illustrated in FIG. 2, the difference between the percent oxygen inthe waste gas and the percent carbon dioxide in the waste gas may alsobe used to determine the phase change of the carbon conversion process.The curve for the difference between the oxygen and carbon dioxide inthe waste gas rapidly increases when the phase change occurs, as isillustrated in FIG. 2. It is desirable, however, due to the gas fluctuations, not to measure the difference in the percent oxygen and carbondioxide in the waste gas until the conversion process has progressed fora predetermined period of time, for example in FIG. 2 after theconversion process has progressed for about 14 minutes, after which themeasurement of the difference between the oxygen and carbon dioxide inthe waste gas is continually measured and plotted against the totalelapsed time of the conversion process. The carbon content of the moltenmetal at the termination of the conversion process may be determined insubstantially the same manner as described above.

The information from the gas analyzer may be transmitted to a computerwhich, in turn, will translate the information and provide a continuousplot of the percent oxygen and percent carbon dioxide in the waste gasand quickly indicate when the phase change occurs in the conversionprocess. The computer can also be utilized to summate the carbon evolvedin the waste gas after the phase change has occurred and continuallyequate and subtract the summated carbon evolved to periodicallydetermine the remaining carbon in the moltenmetal bath so that theconversion process may be terminated when the carbon content of themolten-metal bath reaches a desired preselected level. Also, thecomputer can determine the volume of oxygen required after the phasechange occurs to react with the carbon in the molten-metal bath andremove sufficient additional carbon to provide a desired carbon level atthe termination of the conversion process. After the predeterminedvolume of oxygen is supplied through the lance, the conversion processis terminated.

The following example illustrates the manner of controlling the carboncontent of a molten-metal bath.

EXAMPLE I A charge comprising the following constituents was fed to aconverter:

hot metal 240,800 lbs. scrap metal 108,600 lbs. fluxes 37,710 lbs.

By prior determination it was noted that the carbon phase change in themolten metal occurred when the molten metal contained 0.330 percentcarbon.

An oxygen lance was lowered into the converter and oxygen was deliveredthrough the lance at a constant rate. After approximately 14 minutes thegas analyzer was actuated to periodically measure at S-second intervalsthe total volume of the waste gas in the stack, the volume of carbondioxide in the waste gas and the volume of the unreacted oxygen in thewaste gas in the stack. Other measurements were taken but were notpertinent to the instant invention. The following table illustrates theperiodic measurements. The time was measured from the start of theconversion and the volume of the gases at standard temperature andpressure were determined and summated at 5-second intervals.

The above information was transmitted to a computer and the computerconverted the above information into volume percent and plotted theconverted values against the lapsed time of the conversion process toprovide a graphical representation, as illustrated in FIG. 1. It isapparent at approximately 15.58 a phase change occurred in the carbonreduction process and the molten-metal bath at that time contained 0.330percent carbon.

It was also desired to provide an end product having 0.036 percentcarbon and the volume of oxygen to remove an additional 0.294 percent(0.330%0.036%) carbon from the bath was determined by the computer andthe time required to supply this volume of oxygen at a predeterminedflow rate determined to be 4 minutes from the time of the phase change.The conversion process wascontinued for approximately 4 minutes toprovide a lapsed time of 20.00 minutes at which time the desired carbonlevel of 0.036 percent had been reached and the conversion process wasterminated.

After the phase change had occurred, i.e. 15.58 minutes had elapsed inthe conversion process from start of the oxygen blow, the volume of thecarbon dioxide and the volume of the carbon monoxide in the stack wastegases were carefully recorded by the analyzer and transmitted to thecomputer and the carbon content of the gases leaving'the converter werecontinually summated, equated and subtracted from the known carbon levelat the time of the phase change and the computer indicated when themolten-metal bath had reached a carbon level of 0.036 percent. Onturndown, the product was chemically analyzed for carbon. The accuracyof the above process has proven well within 10.05 percent carbon.

Also, as illustrated in FIG. 1, the percent of carbon monoxide leavingthe converter, as distinguished from the waste gases in the stack, inconcurrence with equilibrium calculations decreases sharply after thephase conversion has taken place.

EXAMPLE II A similar conversion process was carried out where thedifference between the percent oxygen and the percent carbon dioxide inthe waste gases was plotted after the conversion process had been inprogress about 14 minutes. At about 16 minutes the value for the abovedifference between the oxygen and carbon monoxide gases increasedsharply and continuously indicating that the phase change had occurredand 0.330 percent carbon was then present in the molten metal.

According to the provisions of the patent statutes, I have explained theprinciple, preferred construction, and mode of operation of my inventionand have illustrated and described what I now consider to represent itsbest embodiment.

lclaim:

l. A method for determining when a molten-metal bath in a particular topblown oxygen converter contains a predetermined amount of carboncomprising,

a. feeding oxygen to a converter containing a molten-metal bath at apredetermined rate,

b. periodically measuring the amount of unreacted oxygen in the wastegas,

c. periodically measuring the amount of carbon dioxide in the waste gas,

d. noting the time when there is a simultaneous and continuous increasein the amount of unreacted oxygen and a continuous decrease in theamount of carbon dioxide in the waste gas,

e. thereafter measuring the total amount of carbon removed from themolten-metal bath until the conversion process is terminated,

f. determining the total carbon content of the molten-metal bath afterthe conversion process is terminated, and

g. determining from the values obtained by steps (e) and (f) the carboncontent of the molten-metal bath at the time during the conversionprocess when there is a simultaneous and continuous increase in theamount of unreacted oxygen and decrease in the amount of carbon dioxidein the waste gas.

2. A method of determining when a molten-metal bath in a particular topblown oxygen converter contains a predetermined amount of carbon as setforth in claim 1 in which the carbon content of the molten-metal bath atthe time there is a simultaneous and continuous increase in the amountof unreacted oxygen and decrease in the amount of carbon dioxideexpressed as CPC is determined from the relationship where CF is thetotal carbon content of the molten-metal bath after the conversionprocess is terminated and CWG is the summation of the carbon removedfrom the molten-metal bath after the simultaneous and continuousincrease in the amount of unreacted oxygen and decrease in the amount ofcarbon dioxide in the waste gas has occurred.

3. A method for controlling the carbon content of a moltenmetal bath ina top blown oxygen converter comprising,

a. feeding oxygen to a converter containing a molten-metal bath at apreselected rate,

b. measuring the amount of unreacted oxygen in the waste c. measuringthe amount of carbon dioxide in the waste gas,

d. determining the time the phase change occurs in the carbon oxygenreaction by noting the time when a simultaneous and continuous increasein the amount of unreacted oxygen and a continuous decrease in theamount of carbon dioxide in the waste gas occurs,

e. determining the carbon content of the molten-metal bath at the timethe phase change occurs in the carbon reaction from the previous metalconversions in the same converter by measuring in the previous metalconversions the total amount of carbon removed from the molten-metalbath from the time the phase change occurs until the conversion processis terminated, determining the total carbon content of the molten-metalbath in the same previous metal conversions after the conversion processis terminated and determining from the carbon content values the carboncontent of the molten-metal bath at the time during the conversionprocess that the phase change occurred,

f. feeding oxygen to the converter to remove additional carbon from themolten-metal bath,

g. continuously summating from the waste gases the amount of carbonremoved from the molten-metal bath after the phase change occurs,

h. continuously reducing the amount of carbon present in themolten-metal bath at the time the phase change occurred by the summatedamount of carbon removed from the molten-metal bath after the phasechange occurred, and

i. terminating the conversion process when the carbon content of themolten-metal bath reaches a preselected value.

4. A method for controlling the carbon content of a moltenmetal bath ina top blown oxygen converter as set forth in claim 3 which includes,

determining the volume of oxygen required to react with the carbon inthe molten metal after the phase change occurs to remove a predeterminedamount of carbon from the molten-metal bath and reduce the carboncontent to a preselected value,

feeding said predetermined volume of oxygen to the converter to removesaid predetermined amount of carbon, and

terminating the conversion process after said volume of oxygen has beendelivered to said converter.

5. A method for controlling the carbon content of a moltenmetal bath asset forth in claim 3 which includes,

a. measuring the amount of unreacted oxygen in the waste gas,

b. measuring the amount of carbon dioxide in the waste gas,

and

c. determining the time the phase change occurs in the carbon-oxygenreaction by noting the time the difference between the amount ofunreacted oxygen and the amount of carbon dioxide in the waste gasincreases substantially and continuously.

6. A method for controlling the carbon content of a moltenmetal bath asset forth in claim 3 which includes,

measuring the amount of carbon removed from the moltenmetal bath afterthe phase change occurs.

8. A method for controlling the carbon content of a moltenmetal bath ina top blown oxygen converter as set forth in claim 3 which includes,

terminating the conversion process when the molten metal bath containsbelow 0.30 percent carbon.

2. A method of determining when a molten-metal bath in a particular topblown oxygen converter contains a predetermined amount of carbon as setforth in claim 1 in which the carbon content of the molten-metal bath atthe time there is a simultaneous and continuous increase in the amountof unreacted oxygen and decrease in the amount of carbon dioxideexpressed as CPC is determined from the relationship CPC CF+CWG where CFis the total carbon content of the molten-metal bath after theconversion process is terminated and CWG is the summation of the carbonremoved from the molten-metal bath after the simultaneous and continuousincrease in the amount of unreacted oxygen and decrease in the amount ofcarbon dioxide in the waste gas has occurred.
 3. A method forcontrolling the carbon content of a molten-metal bath in a top blownoxygen converter comprising, a. feeding oxygen to a converter containinga molten-metal bath at a preselected rate, b. measuring the amount ofunreacted oxygen in the waste gas, c. measuring the amount of carbondioxide in the waste gas, d. determining the time the phase changeoccurs in the carbon oxygen reaction by noting the time when asimultaneous and continuous increase in the amount of unreacted oxygenand a continuous decrease in the amount of carbon dioxide in the wastegas occurs, e. determining the carbon content of the molten-metal bathat the time the phase change occurs in the carbon reaction from theprevious metal conversions in the same converter by measuring in theprevious metal conversions the total amount of carbon removed from themolten-metal bath from the time the phase change occurs until theconversion process is terminated, determining the total carbon contentof the molten-metal bath in the same previous metal conversions afterthe conversion process is terminated and determining from the carboncontent values the carbon content of the molten-metal bath at the timeduring the conversion process that the phase change occurred, f. feedingoxygen to the converter to remove additional carbon from themolten-metal bath, g. continuously summating from the waste gases theamount of carbon removed from the molten-metal bath after the phasechange occurs, h. continuoUsly reducing the amount of carbon present inthe molten-metal bath at the time the phase change occurred by thesummated amount of carbon removed from the molten-metal bath after thephase change occurred, and i. terminating the conversion process whenthe carbon content of the molten-metal bath reaches a preselected value.4. A method for controlling the carbon content of a molten-metal bath ina top blown oxygen converter as set forth in claim 3 which includes,determining the volume of oxygen required to react with the carbon inthe molten metal after the phase change occurs to remove a predeterminedamount of carbon from the molten-metal bath and reduce the carboncontent to a preselected value, feeding said predetermined volume ofoxygen to the converter to remove said predetermined amount of carbon,and terminating the conversion process after said volume of oxygen hasbeen delivered to said converter.
 5. A method for controlling the carboncontent of a molten-metal bath as set forth in claim 3 which includes,a. measuring the amount of unreacted oxygen in the waste gas, b.measuring the amount of carbon dioxide in the waste gas, and c.determining the time the phase change occurs in the carbon-oxygenreaction by noting the time the difference between the amount ofunreacted oxygen and the amount of carbon dioxide in the waste gasincreases substantially and continuously.
 6. A method for controllingthe carbon content of a molten-metal bath as set forth in claim 3 whichincludes, measuring the amount of carbon removed from the molten-metalbath after the phase change occurs.
 7. A method for controlling thecarbon content of a molten-metal bath in a top blown oxygen converter asset forth in claim 3 which includes, initiating the measurement of theamount of unreacted oxygen and the amount of carbon dioxide in the wastegas after the conversion process has progressed a preselected period oftime.
 8. A method for controlling the carbon content of a molten-metalbath in a top blown oxygen converter as set forth in claim 3 whichincludes, terminating the conversion process when the molten metal bathcontains below 0.30 percent carbon.